This invention relates to an external threaded nut for adjusting the tension of a coil spring, and, more particularly, to a nut which can be formed from a flat strip of material.
The manufacturing of springs is a well-established science. However, because of a multitude of variables, such as tensile strength of the spring material, tolerances of wire size, etc., commercial tolerances for spring loads are generally rather substantial, e.g., of the order of .+-.10%. The tolerances of the spring plus the tolerances of the components which are attached to the spring make it desirable at times to have some adjusting means for compensating for these variables. Adjusting means are also desirable for enabling a particular spring in a particular application to be used with different loads or to vary the spring rate.
It is known to use externally threaded nuts for adjusting the tension of a coil spring. The nut is screwed into one end of the spring so that the spring force is determined by the distance between the nut and the other end of the spring. However, externally threaded nuts in which the threads are cut by screw machinery can be relatively expensive for certain applications.
Automobile trunk lids have been equipped with gas springs to provide a raising force which assists in opening the trunk lid. Gas springs have not been entirely satisfactory, and a coil spring system has been considered by at least one automobile manufacturer. A spring is used because of weight variation of different trunk lids, and the spring tension must be adjustable without substantially increasing the cost of the spring system.
The invention provides an internal adjusting nut for a coil spring which can be manufactured economically and accurately from a flat strip of material by simple stamping and forming operations. The strip is notched to form a disc having a generally circular rim with at least one notch in the periphery of the rim. The rim is formed into a helical shape to provide a portion of one turn of a screw thread which extends around the periphery of the rim from one side of the notch to the other side of the notch. If desired, the center of the disc can be extruded and tapped to receive a threaded rod or bolt. The nut is screwed into a coil spring by inserting the helical rim of the nut between the first and second coil of the spring and rotating the nut with respect to the spring for at least one more turn.